Correspondence

Pulmonary Embolism Associated with Air Travel

N Engl J Med 2002; 346:138-139January 10, 2002

Article

To the Editor:

Dr. Lapostolle and colleagues (Sept. 13 issue)1 offer the best descriptions to date of the epidemiology of pulmonary embolism associated with air travel and of flight distance as a risk factor. There is a problem, however, with the data presented in Table 1 that are used to stratify flight distance.

The distance between the East Coast and the West Coast of the United States is approximately 3000 mi, or just under 5000 km. The United States is listed in the first column of Table 1, “Country or Area of Origin of Flight,” only for the flight distance of 5000 to 7499 km; in fact, although some sites in the eastern United States are within this range of distance from Paris, others in the western United States are more distant and should be represented in the 7500-to-9999-km category of flight distance. The five cases of pulmonary embolism on flights that originated in the United States occurred on flights that originated in the western United States — four in Los Angeles (a distance of 9080 km) and one in San Francisco (8950 km). Since the stratification of cases is based on distance, these five cases appear to be misclassified. The number of cases of pulmonary embolism for the 5000-to-7499-km flight distance should be 4 and the incidence per million passenger arrivals 0.18, and for the 7500-to-9999-km flight distance, the number should be 38 and the incidence 3.07. These updated rates suggest that the risk of pulmonary embolism does not really increase until the flight distance becomes greater than 7500 km, which may be a more appropriate conclusion for the study.

Robert C. McDonald, M.D.
Indiana University School of Medicine, Indianapolis, IN 46204
bob.md.mcdonald@anthem.com

1 References

1. 1

   Lapostolle F, Surget V, Borron SW, et al. Severe pulmonary embolism associated with air travel. N Engl J Med 2001;345:779-783
   Full Text | Web of Science | Medline

Author/Editor Response

The authors reply:

To the Editor: In the case of the passengers arriving at Charles de Gaulle Airport from the United States, the exact origin of all the passengers — that is, the city — was not available. We thus chose to classify all the flights from the United States in the 5000-to-7499-km range of distance. Since the hypothesis of the study was that the incidence of pulmonary embolism was correlated with the distance traveled, our classification was, a priori, not favorable to the tested hypothesis.

We agree with Dr. McDonald that this choice may have introduced a misclassification. However, we strongly disagree with his alternative method of calculation. In effect, he has reclassified the patients with pulmonary embolism arriving from the United States into the 7500-to-9999-km range of distance without taking into account the (unknown) number of passengers in the denominator who would need to be reclassified into the same group. Furthermore, even if one accepted the recalculation he proposes, it would not modify the shape of the graph of the incidence of pulmonary embolism shown in Figure 1 of our article. There is clearly a transition in the incidence in the 5000-to-9999-km range of distance. But the exact distance corresponding to the point of transition has not been determined.

We believe that the conclusion should be that the risk increases with travel of more than 5000 km. This threshold is in accordance with the method of classification we used and is applicable to our discussion about the methods for preventing thromboembolic events. In contrast, the conclusion of Dr. McDonald, suggesting that the risk increases with travel of more than 7500 km, ignores five patients who had pulmonary embolism after travel (from Senegal, India, Angola, or the Netherlands Antilles) of less than 7500 km. Such a conclusion might introduce an underestimation of the risk and result in failure to introduce appropriate prophylactic measures.

Frédéric Lapostolle, M.D.
Stephen Borron, M.D.
Hôpital Avicenne, 93009 Bobigny, France
frederic.lapostolle@avc.ap-hop-paris.fr